This invention relates to a coupling for interconnecting a control shaft, a driven shaft and a servo-motor which normally powers the driven shaft. This coupling is particularly useful in an automotive vehicle power steering system. It is an improvement on my invention set forth in my prior patent application U.S. Ser. No. 392,993, filed June 28, 1982 now U.S. Pat. No. 4,458,795 entitled "COUPLING FOR DRIVE SHAFT AND SERVO-MOTOR DRIVEN SHAFT"
In general, a power steering system for an automotive type vehicle includes a control shaft or steering column which is controlled by the vehicle operator turning the steering wheel, and a power assist device for the driven steering system. Hydraulic devices are conventionally used to provide the power assist. In addition, it is known to use a servo-motor arrangement to provide the power assist for the drive. An example of the use of a servo-motor arrangement is disclosed in U.S. Pat. No. 3,893,534 issued July 8, 1975 to Steinmann.
In any power assist device used in a power steering system, there is a danger of malfunctioning or power failure. Although failures are rare, it is necessary to provide a direct drive alternative to the power assist drive during times of malfunctioning or failure. When the servo-motor resists or opposes the action of the driver, the servo-motor should be disengaged, whereas it should remain engaged if it provides some assistance even though it may be in a state of partial failure. Before my invention set forth in U.S. Ser. No. 392,990, systems lacked these qualities to some degree or had other deficiencies which kept them from achieving commercial success.
Thus, this invention describes an improved coupling which provides a fail-safe connection between the servo-motor power source and the control and driven shafts or members to allow a direct drive in the event of power failure. During such direct drive the servo-motor power source is prevented from interfering with, but is allowed to assist the direct drive.